In the plastics or synthetic-resin arts, it is a common practice to apply a contoured pattern to a surface of a synthetic-resin plate, sheet or foil using an embossing tool having the complementary contour or pattern.
The pattern-forming operation may result from a molding of the pattern against the tool or by pressing the tool into the preformed plate or web of sheet or foil material.
The tool, which can be a metal drum or plate, can thus form part of an embossing press, e.g. a multideck or multilevel press, or can form part of a mold in which the synthetic-resin material is shaped.
The application of an embossing pattern to synthetic-resin materials has been used to emboss relatively soft synthetic-resin sheets which can be used in the upholstery, luggage, show or garment industries. In this case, the synthetic-resin sheet has an embossed pattern which may correspond to the pattern of natural leather, textile fabrics and the like. In this case, the synthetic-resin sheet or foil, generally as a continuous web, is passed between an embossing roll and a counterpressure roll, the embossing roll having a metal surface in which the complementary pattern is formed.
Plastic plates used in the furniture industry can have a grid-like rough wood-imitation pattern, and are usually formed by molding in presses having press plates constituting the tool and provided with a surface texture or contour complementary to that to be reproduced in the product.
In all cases, it is important that the molding tools produce an uniform pattern on the plastic plates or sheets without join marks, steps or lines between portions of the pattern.
This is necessary to ensure that the imitation of the natural product, such as wood or leather, will be as authentic as possible. In addition, it is essential that one be able readily to produce a multiplicity of embossing tools which are absolutely identical so that worn tools can be replaced. Large numbers of identical tools are also required when multideck presses are to be used with a plurality of such tools in the mass production of large numbers of embossed identical articles.
Thus, in this field, it is important to be able to produce high-quality, identically-patterned metal embossing tools in an inexpensive manner.
To produce embossing patterns on embossing tools for the aforedescribed purpose, several methods are known. Direct engraving by hand using conventional engraving tools and even machine engraving is impractical because the process is usually extremely time-consuming and does not permit of exact reproduction of the desired pattern.
Thus, the preparation of such tools has required reliance upon etching techniques. In the etching approach, an etch resist is applied to the metallic surface of a plate or roll to constitute the tool and areas not covered by the resist are subjected to treatment with an etching solution and chemical removal by solubilization of the metal.
There are two main techniques for generating an etch-resist pattern upon the metal surface. In the reproduction photography approach, the resist is a photosensitive material which is exposed through a photographic negative. The unfixed portions of the resist are washed away while the fixed portions remain so that etching occurs in the regions in which the resist has been removed. The other approach applies the resist in a predetermined pattern by screen printing.
Some problems have been encountered heretofore in the production of embossing or molding plates by reproduction photography, for example, at the present time photographic films have a maximum width of 160 cm, for all practical purposes, so that, to form a plate with dimensions of 230.times.600 cm., at least two negatives must be placed upon the substrate coated with the photosensitive resist. Where the negatives adjoin, a step or interruption is formed in the pattern to be reproduced, thereby requiring a difficult, time-consuming machining process to retouching process for removal of the join line. Frequently such attempts are unsatisfactory or incomplete. This disadvantage is most pronounced when fine-grain textures are to be produced since, in this case, the film is formed with a multitude of minute dots.
In the screen-printing technique, wherein the etching resist is applied by means of a rotary screen directly to the surface of the metal plate or sheet, problems are also encountered. The pattern on the rotary screen is often produced by a photographic process so that the difficulties previously described may result here as well. Furthermore, the screen is generally a perforated metal sheet or wire mesh of cylindrical configuration through which a relatively viscous substance, namely, the etching resist, is pressed by a doctor plate onto the metal plate. Because of the viscosity of this material, it does not penetrate fine meshes having 100 or more openings per cm. with sufficient uniformity to allow the reproduction by the etched metal plate of fine-grain textures.
Another characteristic of all prior-art etching techniques for the preparation of embossing plates is that the depths of the etching is the same in all areas while the unetched regions or lands remain completely smooth. Natural-looking imitations of wood grain, with the multilevel texture extending generally across the entire width of the surface, cannot be achieved with techniques which provide a constant depth of etching and perfectly smooth lands over the entire textured surface. Other fine-grain surfaces such as those involved in the production of imitation leather also cannot be produced by embossing plates with smooth-land characteristics. In the latter case, the fine details required for a true imitation of natural products cannot be reproduced in the etching resist or upon the embossing plate because the viscosity of the resist does not normally allow fine and close detail to be readily duplicated. In other words, with screen printing using viscous resist, the resist can cover relatively larger areas of the metal surface than is desirable and as a result the smooth surfaces of lands remaining beneath the resist has a relatively large area, unsatisfactory for fine detailing.